The focus of the proposed work is a 2-oscillator model of the "complex" pacemaking system that drives circadian rhythms in mammals and is causally involved in the photoperiodic control of their reproductive activity. The experiments fall into three general groups. The first exploits a recent technical breakthrough in our laboratory that opens up the model to rigorous testing and development: we can now measure the phase-response-curves (PRCs) of mammalian circadian pacemakers with previously unmatched precision and at a rate (2 PRCs every three weeks) that makes it possible for the first time to track the detailed time-course of the pacemaker which the model predicts is strongly history-dependent. The experiments will go hand in hand with further development of the model by computer simulation techniques. The second group focuses on the role of the circadian pacemaker in the photoperiodic control of reproduction in hamsters. The third group of experiments concern the function of the suprachiasmatic nuclei (can the constituent oscillators of the complex pacemaker be localized within the SCN?) and the pineal gland (does melatonin mediate the mutual coupling of the constituent oscillators?)